This invention relates to electro-optic devices comprising a photoconductive layer in contact with a liquid crystalline layer; more particularly this invention relates to imaging the device with suppressed premature response by the liquid crystalline layer prior to imaging.
Electro-optic devices comprising a photoconductive layer and liquid crystalline layer are known. Typically, the liquid crystalline layers respond by changing optical characteristics in response to some threshold level of voltage. In imaging, a voltage is applied across both the photoconductive and liquid crystalline layers in an amount insufficient to produce response by the liquid crystalline layer. Imagewise actinic radiation is generally utilized to render the conductor conductive in imagewise configuration. This causes the voltage across the photoconductor to decrease and the voltage across the liquid crystalline layer to increase, in corresponding imagewise configuration. Typically, the voltage increases across the liquid crystalline layer to a level at or above the threshold level of voltage to which the liquid crystalline layer responds thereby causing a changing in optical properties in imagewise configuration.
A difficulty frequently encountered in imaging electro-optic cells comprising a photoconductive layer in contact with a liquid crystalline layer is that prior to imaging, and merely upon applying a voltage to the cell, premature response by the liquid crystalline layers occurs and a significant amount of the liquid crystalline layer is unavailable for imaging and often interferes with subsequently formed images. This difficulty is also often encountered during removal of the applied field after imaging.
The manner in which a voltage is applied to a liquid crystalline layer is of consequence as indicated by U.S. Pat. Nos. 3,503,673; 3,519,330; 3,575,491 and 3,575,493. In general, these patents are directed to the rapid onset and removal of dynamic scattering in electro-optic cells containing nematic liquid crystalline materials but lacking a photoconductive layer.
Improved electro-optic effects are obtained for an electro-optic cell containing a photoconductive layer in contact with a liquid crystalline layer in U.S. Pat. No. 3,732,429 wherein a layer of high electrical impedance is included in the cell. This provides a higher dark impedance to the cell without significantly affecting the light-to-dark impedance ratio so that, upon imaging with actinic light, the contrast between scattered and non-scattered areas of the imaged cell is enhanced.
In new and growing areas of technology, new methods, apparatus, compositions, and articles of manufacture are often discovered for the application of the new technology in a new mode. The present invention relates to a new and advantageous method of imaging an electro-optic cell comprising a photoconductive layer in contact with a liquid crystalline layer.